Cell phones don’t work on the 2 metre band. That’s the amateur radio band, and fire and ambulance frequencies. The 4 metre band is half that again, about 75 MHz, just below FM radio.

Analog cell phones are up around 900 MHz, which has a wavelength of about 35 cm. To jam them, you’ll have to build a transmitter for each channel.

Digital cell phones are harder to jam, as you’d have to build a pseudo-random noise generator in the microwave region of the band (1.8 – 1.9 GHz)

You may have a chance of swamping the local oscillator, if you can identify the IF frequency inside the cell phone, and then generate that signal.

I trust you will only use this inside a Faraday cage, for educational pursuits, otherwise, without the appropriate license (for which you’d already know the answer to the question) you’ll have the Canadian IC or USA FCC knocking on your door if you live in North America.